RE: mdadm raid1 read performance

["Leslie Rhorer" <lrhorer@satx.rr.com>]

> -----Original Message-----
> From: linux-raid-owner@vger.kernel.org [mailto:linux-raid-
> owner@vger.kernel.org] On Behalf Of Keld Jørn Simonsen
> Sent: Thursday, May 05, 2011 6:10 AM
> To: NeilBrown
> Cc: Liam Kurmos; Roberto Spadim; Brad Campbell; Drew; linux-
> raid@vger.kernel.org
> Subject: Re: mdadm raid1 read performance
> 
> On Thu, May 05, 2011 at 09:45:38AM +1000, NeilBrown wrote:
> > On Thu, 5 May 2011 00:08:59 +0100 Liam Kurmos <quantum.leaf@gmail.com>
> wrote:
> >
> > > as a separate question, what should be the theoretical performance of
> raid5?
> >
> > x(N-1)
> >
> > So a 4 drive RAID5 should read at 3 time the speed of a single drive.
> 
> Actually, theoretically, it should be more than that for reading, more
> like N minus
> some overhead. In a raid5 stripe of 4 disks, when reading you do not read
> the checksum block, and thus you should be able to have all 4 drives
> occupied with reading real data. Some benchmarks back this up,
> http://home.comcast.net/~jpiszcz/20080329-raid/
> http://blog.jamponi.net/2008/07/raid56-and-10-benchmarks-on-26255_10.html
> The latter reports a 3.44 times performance for raid5 reads with 4
> disks, significantly over the N-1 = 3.0 mark.
> 
> For writing, you are correct with the N-1 formular.

	There have been a lot of threads here about array performance, but
one important factor rarely mentioned in these threads is network
performance.  Of course, network performance is really outside the scope of
this list, but I frequently see people talking about performance well in
excess of 120MBps.  That's great, but I have to wonder if their network
actually can make use of such speeds.  Of course, if the application
actually obtaining the raw data is on the machine, then network performance
is much less of an issue.  A database search implemented directly on the
server, for example, can use every bit of performance available to the local
machine.  Given that in my case the vast majority of data is squirted across
the LAN (e.g., these are mostly file servers), anything much in excess of
120MBps is irrelevant.  I mean, yeah, it’s a rather nice feeling that my
RAID arrays can deliver more than 450MBps if they are ever called upon to do
so, but with a 1G LAN, that's not going to happen very often.  I just wonder
how many people who complain of poor performance can really benefit all that
much from increased performance?

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